Power supplies for modern electronic circuits are typically required to provide stable regulated supply voltages for proper operation of processors, ASICS, memory, and other components. Computers, smart phones, tablets, and other electronic products typically employ DC to DC converters to provide supply voltages for operation of the internal circuitry. During operation, however, the current required to operate various circuit components changes, and a power supply may need to regulate the supply voltage within a narrow tolerance band (e.g., +/−3% or less) even in the presence of large current draw variations over a short period of time. The variation in current draw is reflected as a load transient to the DC to DC converter that provides the supply voltage, and DC to DC converter controller performs closed loop converter operation to maintain a generally constant output voltage during these load transients.
DC to DC converters typically include one or more switching devices operated by pulse width modulated switching control signals, and a common form of pulse width modulation (PWM) employs a generally fixed switching frequency with the controller modifying the width or duration of the switching control signal pulses to regulate the output voltage according to a feedback signal to compensate for changes in the output load requirements. Fixed frequency DC to DC conversion, however, suffers from voltage regulation limitations in the presence of abrupt changes in load current. In particular, many DC to DC converter architectures employ an output inductor providing current to the load, and switching operation of the converter switch or switches selectively connects the inductor to the source of input power during the “on time” of the PWM switching signal pulses. If the load current increases quickly, particularly when the power supply switch is in an “off” state, the inductor energy may become depleted until the next switching interval. As a result, the output voltage may decrease significantly or “dip”. Subsequent PWM switching pulses can replenish the inductor current, but the closed loop control in a fixed frequency control approach may not be able to prevent a significant voltage deviation in the time it takes to react, and subsequent switching operation may cause an overshoot in the output voltage as the circuit returns to regulated control. The initial voltage drop following an increase in the required load current is referred to as the load transient response of the DC to DC converter. The magnitude of the voltage dip may be addressed, to some extent, by addition of output capacitance to source the energy demanded during the load transient, but additional capacitance increases the cost and size of the power conversion system. Variable frequency PWM approaches may be adopted in order to address load transient response issues, including hysteretic constant on-time, constant off-time, and other approaches, which may in certain circumstances closely approximate fixed frequency operation, but variable frequency designs may be difficult to implement for use in systems synchronized with a system clock or in multiphase or stacked converter configurations. The use of hysteretic comparators providing a control loop outside of the fixed frequency inner loop, moreover, requires the controller to multiplex an I/O pin for monitoring the output voltage and the selection of filter components to avoid loop-to-loop oscillations is limited by the use of the comparators. Accordingly, a need remains for improved fixed frequency pulse width modulation control apparatus and techniques for improved regulation and reduced voltage dip magnitude in the presence of abrupt increases in load current requirements.